#include <bits/stdc++.h>
using namespace std;
#define int long long

signed main() {
  string s;
  cin >> s;

  int N = s.size();
  map<int, int> m;
  int p = 0;
  m[0] = -1;

  int ans = 0;
  for (int i = 0; i < N; i++) {
    char c = s[i];
    if (c == 'A')
      p++;
    else
      p--;

    if (m.count(p)) ans = max(ans, i - m[p]);
    if (!m.count(p)) m[p] = i;
  }

  cout << ans << endl;
  return 0;
}